1. Field of the Invention
The present invention relates to a method that may arrange an optical phase modulator outside an interferometer and may modulate an optical phase in a phase modulation based quantum key distribution system.
2. Description of the Related Art
A quantum key distribution system may load key information on a single photon and may transmit the single photon to a receiver by adjusting a polarization or a phase of the single photon. A receiver may extract the key information using a polarization receiver, an optical phase modulator, and the like. The single photon transmission may be embodied based on optical communication technologies, and the quantum key distribution system intending to perform long-distance transmission may use a single mode optical fiber as a quantum channel. When a polarization-modulated single photon is transmitted through the single mode optical fiber, a polarization feature becomes unstable and thus, a phase modulation scheme is preferred for the key distribution than the polarization modulation scheme.
The phase modulation-based quantum key distribution system may mainly use a time-division optical interference scheme. An asymmetric optical interferometer and an optical phase modulator, and the like may be used to perform the time-division optical interference. The asymmetric optical interferometer may have two paths having different lengths, and the two paths may be used for an optical interference. A probability of existence of a single photon inputted to the asymmetric optical interferometer may be divided into two distributions of which probabilities of existence of the single photon have different coordinates in a time domain. The optical phase modulator modulates a phase of a single photon passing through one of the two paths. An asymmetric optical interferometer of the receiver may divide a probability of existence into four coordinates in the time domain. When a path difference of an asymmetric optical interferometer of a transmitter and a path difference of the asymmetric optical interferometer of the receiver are the same, adjacent two probabilities among the four probabilities of existence of the single photon may overlap with each other and may cause interference. The receiver may include an optical phase modulator, and the optical phase modulator may modulate a phase of the single photon. When a sum of phases modulated by the transmitter and the receiver is 2nπ, n being a natural number, the overlapped two probabilities of existence of the single photon may show a maximal detection probability by constructive interference, and when a sum of a sum of phases modulated by the transmitter and the receiver is (2n+1)π, the overlapped two probabilities of existence of single photon may shows a minimal detection probability by destructive interference.
A stability with respect to a polarization and a phase feature of the optical interferometer may be secured to obtain an excellent optical interfering performance. Two single photons interfering with each other may have the same polarization, and a phase with respect to an entire optical path, excluding a phase modulation value additionally provided by the optical phase modulator, may be maintained.
An optical interferometer for a conventional phase modulation based-quantum key distribution system may arrange the optical phase modulator inside an interferometer path.
FIG. 1 illustrates an example of a configuration of a conventional optical interferometer. A quantum key distribution system may include a transmitting-end 110, a channel 120, and a receiving-end 130. The transmitting-end 110, the channel 120, and the receiving-end 130 may be connected with each other. The optical interferometer may be a Mach-Zehnder type interferometer. The optical interferometer may receive a photon from a light source 101, and a delay line 103 and an optical phase modulator 104 used for constituting an asymmetric optical interferometer may be arranged between a beam splitter 102 and a beam splitter 105. In the receiving-end 103, a delay line 133 and an optical phase modulator 134 used for constituting the asymmetric optical interferometer may be arranged between a beam splitter 132 and a beam splitter 135. Photon detection may be performed by two single photon detectors 138 and 139. A high-speed optical phase modulator may be used for high-speed transmission of a quantum key. The high-speed optical phase modulator may be manufactured based on a LiNbO3-based planar lightwave circuit scheme. Both ends of a LiNbO3-based optical phase modulator may be pigtailed with optical fibers. When an optical modulator to which a pigtailed optical fiber is attached is inserted to the optical interferometer, a length of an optical path is extended and thus, instability of the optical interferometer may increase, and a configuration may become complex. The optical interferometer may be configured based on a polarization-dependent feature of the planar lightwave circuit and thus, difficulty in configuring the optical interferometer may increase.
The instability of the asymmetric optical interferometer due to the extended optical path may be caused by the following:
An optical fiber-based optical interferometer may be sensitive to a change in vibration and temperature. Specifically, an optical fiber may have a change in an effective length, as shown in Equation 1, due to a thermo-optic coefficient and a thermal expansion coefficient. As a length is extended and becomes longer, the change in the effective length may be higher.
                                          ⅆ                          (              nl              )                                            ⅆ            T                          =                                            l              ⁢                                                ⅆ                  n                                                  ⅆ                  T                                                      +                          n              ⁢                                                ⅆ                  l                                                  ⅆ                  T                                                              =                      nl            ⁡                          (                                                                    1                    n                                    ⁢                                                            ⅆ                      n                                                              ⅆ                      T                                                                      +                                                      1                    l                                    ⁢                                                            ⅆ                      l                                                              ⅆ                      T                                                                                  )                                                          [                  Equation          ⁢                                          ⁢          1                ]            
In Equation 1, n may denote an effective refractive index, l may denote a length of the optical path, and T may denote a temperature. dn/dT may denote a thermo-optic coefficient and 1/l*dl/dT may denote a thermal expansion coefficient. An amount of change in an effective length of an optical path with respect to an asymmetric optical interferometer of transmitter and an amount of change in an effective length of an optical path with respect to an asymmetric optical interferometer of the receiver may be different, the transmitter and the receiver being in different environments. Therefore, relative phases of two single photons may not be maintained to be a predetermined value.
Unlike the example of FIG. 1, when a Michelson interferometer is used, the length of the optical path may be extended and become longer and thus, the instability may be higher. The optical phase modulator may basically include an insertion loss of several dBs and thus, a loss difference between two asymmetric paths may be relatively higher when the Michelson interferometer is applied.
Various schemes may be attempted to overcome difficulties of a conventional scheme. One attempt is decreasing of a length of a pigtailed optical fiber. However, there is limit to the decreasing of the length, since a predetermined length may need to be secured when an optical path may be constructed using an optical connector and an optical fiber fused connection, and the like. Another attempt is to not perform of pigtailing to the optical phase modulator. However, when the optical phase modulator to which the pigtail is not performed is applied to the optical fiber-based interferometer, a complex optical alignment problem may be incurred.